They have, but I haven't used navigation really much to say if they really work well. Both do have turn by turn navigation with voice. You'd better just try them. A good thing about Mappero and modRana is, they use same directory and files as a offline storage for maps, so when you download with one application the maps you need, you can use the maps with both apps.

The problem with both are that they currently need network connection to find a route from place A to B. But both are still developed and maybe get that feature too, offline routing. However you can find routes online beforehand, and save them to later use in offline mode.

The biggest problem using N900 offline for location is the need of A-GPS. GPS without A-GPS is disgusting slow.